Shocks and Slip Systems: Predictions from a Mesoscale Theory of Continuum Dislocation Dynamics

ARTICLE IN PRESS Journal of the Mechanics and Physics of Solids 56 (2008) 1450–1459 www.elsevier.com/locate/jmps Shocks and slip systems: Predictions from a mesoscale theory of continuum dislocation dynamics S. Limkumnerda,Ã, J.P. Sethnab aZernike Institute for Advanced Materials, University of Groningen, 9747 AG, Groningen,The Netherlands bLaboratory of Atomic and Solid State Physics, Clark Hall, Cornell University, Ithaca, NY 14853-2501, USA Received 4 February 2007; received in revised form 18 July 2007; accepted 26 August 2007 Abstract Exploring a recently developed mesoscale continuum theory of dislocation dynamics, we derive three predictions about plasticity and grain boundary formation in crystals. (1) There is a residual stress jump across grain boundaries and plasticity-induced cell walls as they form, which self-consistently acts to attract neighboring dislocations; residual stress in this theory appears as a remnant of the driving force behind wall formation under both polygonization and plastic deformation. We derive the predicted asymptotic late-time dynamics of the grain-boundary formation process. (2) During grain boundary formation at high temperatures, there is a predicted cusp in the elastic energy density. (3) In early stages of plasticity, when only one type of dislocation is active (single-slip), cell walls do not form in the theory; instead we predict the formation of a hitherto unrecognized jump singularity in the dislocation density. r 2007 Elsevier Ltd. All rights reserved. PACS: 46; 91.60.Dc; 91.60.Ed; 47.40. x À Keywords: Dislocations; Shocks; Burgers equations; Singularity formation; Plasticity 1. Introduction Dislocations in crystals evolve to form structures, especially walls: grain boundaries at high temperatures where climb is allowed, cell boundaries under low-temperature plastic deformation when climb is forbidden. We examine here the detailed predictions of a new variant of continuum dislocation dynamics (Roy and Acharya, 2005; Limkumnerd and Sethna, 2006), that spontaneously forms sharp walls as shock-wave solutions of the partial differential equation (Limkumnerd and Sethna, 2006; Cho, 2006). How does this new approach fit in to the extensive existing literature on continuum plasticity and dislocation structure formation? First, there are many large-scale simulation studies using discrete dislocations (Holt, 1970; Lepinoux and Kubin, 1987; Gullouglu et al., 1989; Ghoniem et al., 1990; Lubarda et al., 1993; Barts and Carlsson, 1997; Koslowski and Ortiz, 2004; Ramasubramaniam et al., 2007). The computational ÃCorresponding author. Tel.: +31 503638044; fax: +31 503634886. E-mail address: [email protected] (S. Limkumnerd). URL: http://www.lassp.cornell.edu/sethna/sethna.html (J.P. Sethna). 0022-5096/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jmps.2007.08.008 ARTICLE IN PRESS S. Limkumnerd, J.P. Sethna / J. Mech. Phys. Solids 56 (2008) 1450–1459 1451 burden of evolving an enormous number of dislocation segments with long-range interactions is a primary motivation for developing a continuum theory, which replaces the explicit dislocation lines with an average tensorial density. Whatever continuum theory succeeds in describing the collective behavior, these discrete dislocation simulations will remain as the validation underpinning the continuum theory and likely also as its connection to specific materials properties. Second, there are theories (along with an earlier, scalar attempt (Sethna et al., 2004) to explain wall formation) which ignore the tensor structure of the dislocation density. (a) Macroscopic continuum plasticity often makes use of the simple von Mises law, which presumes an elastic response when a yield stress is reached, after which the distortion tensor evolves according to the local deviatoric stress (stress with the isotropic pressure removed). Generalizations of the von Mises approach which incorporate corrections due to gradients in the local distortion tensor have grown out of early work on size-dependent hardness (Fleck and Hutchinson, 1993) and dislocation patterns (Aifantis, 1984a, b; Walgraef and Aifantis, 1985; Pontes et al., 2005). (b) Models of plasticity in glasses (a somewhat different physical system) describe localized rearrangements of atoms (Falk and Langer, 1998; Langer, 1998), and appear to generate fractal avalanches and crackling noise (Bailey et al., 2007) reminiscent of crackling noise recently observed in crystal plasticity (Zaiser, 2006). (c) There are a variety of reaction– diffusion models which have been used to describe the widths of persistent slip bands and other dislocation patterns (Aifantis, 1984a, b; Walgraef and Aifantis, 1985; Pontes et al., 2005), cellular structures (Kratochvil, 1990a, b), double cross-slip (Bre´ chet and Louchet, 1988), dislocation vein structures (Saxlova´ et al., 1997), and many other effects (Ha¨ hner, 1996). In the domains for which these theories were developed, the dislocation density or its tensor structure can be argued to be largely irrelevant; for example, the isotropic theory of work hardening is a reasonable first approximation to macroscopic plasticity. But by omitting explicit evolution of the dislocation density tensor, these approaches lose the ability to predict the rotational and deformation morphology of the mesoscopic dislocation structures, and they lose the connection between the microscopic Peach–Koehler forces on the dislocations and the resulting continuum dynamics—both crucial properties that we want to incorporate into our theory. Third, there are theories which incorporate more microscopic detail about the dislocation content than we keep, keeping track not only of the net dislocation density but of local dislocation densities for each slip system (including oppositely oriented Burger’s vectors), and incorporating dislocation entanglement as effective hardening rules coupling the densities on different slip systems. These researchers have studied both texture (grain orientation distribution) evolution in polycrystal plasticity and the evolution of subgrain structures, either for their own sake (Mika and Dawson, 1999; Barton and Dawson, 2001; Dawson et al., 2002; Arsenlis and Parks, 2002; Arsenlis et al., 2004; Ma et al., 2006) or as a precursor for other computations (like recrystallization simulations (Raabe and Becker, 2000)). Of these three features missing in our model (canceling ‘geometrically unnecessary’ dislocations, entanglement, and slip systems) the first can plausibly be ignored on the mesoscale. In macroscopic plasticity, most dislocations cancel out in the net density; ignoring the geometrically unneccessary dislocations would be a poor description. On the mesoscale, the crystal misorientations across cell walls and grain boundaries, and the accepted microscopic structure of grain boundaries (Hirth and Lothe, 1992), are solely due to the net dislocation density kept in our model. (Indeed, the geometrically unnecessary dislocations on the macroscale could plausibly be largely due to cancellations between dislocations on mesoscopically separated walls.) The other two features we omit clearly remain important on the mesoscale. Indeed, in the simulations presented here the dislocations evolve in time, where in practice they evolve under increasing strain—a reflection of the lack of entanglement or work hardening in our model. However, our mesoscale theory, by ignoring entanglement and slip planes, does succeed in providing a striking explanation for wall formation that has not emerged analytically from these more detailed theories (although wall formation may have been observed numerically in these models (Mika and Dawson, 1999; Barton and Dawson, 2001; Dawson et al., 2002)). The more detailed and quantitative predictions of the theory presented here must be interpreted as a first approximation, to which the effects of entanglement, slip planes, anisotropy, and materials properties will need to be added. Finally, Ortiz and collaborators have extended the mathematical minimizing-sequence techniques developed for studying martensitic and magnetic microstructures to describe the formation of dislocation microstructures (Ortiz and Repetto, 1999; Ortiz et al., 2000; Aubry and Ortiz, 2003; Conti and Ortiz, 2005). While it is perhaps too early to draw broad generalizations, it would seem that the model presented here ARTICLE IN PRESS 1452 S. Limkumnerd, J.P. Sethna / J. Mech. Phys. Solids 56 (2008) 1450–1459 quantifies the formation of the dislocation walls, and provides a microscopic model for studying their morphology and evolution, whereas the variational methods directly solve for the microstructure and not how it originates. The model we study was originally proposed by Roy and Acharya (2005), who allowed both glide and climb. We subsequently rediscovered this law from the microscopic dynamics and a closure approximation, proposed a modified law to suppress climb, and showed numerically that the models developed shock singularities (Limkumnerd and Sethna, 2006), providing a potential underlying explanation for both the grain boundaries formed at high temperatures and the cell walls formed at low temperatures in plastically deformed crystals. In Section 2 we will present Roy and Acharya’s derivation for the equations of motion. In Section 3 we will analyze the solution of our model in one dimension. Near a locally-flat dislocation wall, the properties may be approximated by a 1D theory, incorporating variations only perpendicular to the wall; hence the

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